Air or oxygen depolarization cells are generally well known in the art. Electro-chemical cells of this type include a gas diffusion cathode capable of generating electricity by electro-chemically combining an oxidizable reactant with a reducible reactant. Generally, these electro-chemical cells are comprised of spaced apart electrodes ionically connected by an electrolyte. Illustrative of these cells and electrodes are U.S. Pat. Nos. 3,649,361, 3,553,024, 3,513,030, and 3,527,616.
Until recently, metal/air cells were not suitable for applications in which a rapid discharge was required. Presently, however, metal/air cells are capable of achieving high energy to density ratios making them useful in rapid discharge applications such as light weight intraurban vehicles.
The air electrodes utilized in present state-of-the-art metal/air cells, however, are unifunctional; that is, they are incapable of electro-chemically regenerating the fuel electrode. Therefore, in order to recharge the battery a third, counter-electrode is required. Since the counter-electrode is used only during the recharging cycle and the air electrode only during discharge, the cell requires a complicated geometric design as well as complex mechanical or electrical switching means. Additionally, the use of a counter-electrode increases the dead weight of the cell thereby reducing the watt-hr./lb. energy storage capacity. In addition to those disadvantages, it has been found that the presence of the counter-electrode reduces the cell voltage, resulting in a deterioration of the capacity of the metal electrode on extended cycling, and provides high internal resistance.
Notwithstanding the disadvantages of metal/air cells having unifunctional air electrodes, these cells are extremely useful in applications where recharging is not required. Many applications require low cost metal/air batteries of the "throw-away" type or in replaceable anode cells.
Accordingly, it is an object of the present invention to provide a bifunctional air electrode for use in metal/air cells that undergo substantion charge-discharge cycling. It is a further object of the invention to provide a bifunctional air electrode which not only eliminates the need for a counter-elelctrode, but which has a long cycling life, stability, and high energy capacity. Another object of the invention is to provide a low cost air electrode having improved operating characteristics for use in metal/air cells, particularly iron/air cells and for use in hydrogen/oxygen fuel cells.